1. Technical Field
The present invention generally relates to automatic gain control circuits, and particularly to an automatic gain control circuit used in digital television receivers.
2. Description of Related Art
Digital televisions have the advantages of higher definition (or higher resolution) and compact disc (CD) level multi-channel audio output as compared to traditional analog televisions. Nowadays, various countries such as United States, Europe and Japan have already established their own digital television broadcast formats, e.g., vestigial sideband (“VSB”) for the United States. The detailed information with respect to the VSB broadcast format has been published in a paper by Wayne et al. on IEEE Transactions on Consumer Electronics, vol. 41, No. 3 (August 1995), entitled “VSB Modem Subsystem Design for Grand Alliance Digital Television Receivers”, the disclosure of which is incorporated herein by reference.
A typical digital television receiver primarily includes an antenna for receiving radio frequency (RF) signals (i.e., generally high frequency signals), a tuner for channel select, an automatic gain control circuit and a demodulator. The automatic gain control circuit generally includes a low noise RF amplifier, a frequency converter, an Intermediate Frequency (IF)/Baseband amplifier, and an analog-to-digital (A/D) converter and an automatic gain controller. The low noise RF amplifier is for amplifying an RF signal of the selected channel. The frequency converter is for converting the selected RF signal into an intermediate frequency (IF) signal/baseband (zero-IF) signal. The IF/Baseband amplifier is for amplifying the IF/Baseband signal to be an amplified IF/Baseband signal. The A/D converter is for converting the amplified IF/Baseband signal into a digital IF/Baseband signal. The digital IF/Baseband signal is outputted to the demodulator for demodulation as well as the automatic gain controller. The automatic gain controller receives the digital IF/Baseband signal, compares the digital IF/Baseband signal with a reference level and then generates gain control signals in analog form to independently control gains of the low noise RF amplifier and the IF/Baseband amplifier, so as to regulate the signal outputted from the IF/Baseband amplifier at a desired level.
For the above-described low noise RF amplifier, when the low noise RF amplifier receives relatively weak RF signals, gain (or magnification) of the RF signals can be increased to improve the noise performance of the digital television receiver. When receiving strong RF signals, a linearity of the gain of RF signals may not accurately adjust and signal distortion may occur. As a result, the performance of the digital television receiver will degrade.
Therefore, what is needed is an automatic gain control circuit which includes a low noise RF amplifier and a gain thereof has an improved linearity and can be accurately adjust.